(1) Field of the Invention
The present invention relates to an improvement in an electrostatic photographic process in which ordinary electrostatic photographic reproduction and electrostatic photographic printing including the step of imagewise exposure conducted once and the charging step conducted many times can be carried out repeatedly by using a single photosensitive material.
(2) Description of the Prior Art
As is well-known, according to the electrostatic photographic process, copies and prints are prepared by forming an electrostatic latent image by the combination of the step of charging a photoconductive photosensitive layer with charges of a certain polarity and the steps of subjecting the photoconductive photosensitive material to imagewise exposure, developing the formed electrostatic latent image with a toner such as a detecting powder, transferring the toner image to a copy sheet and, if necessary, fixing the transferred toner image.
In this electrostatic photographic process, there is known a method in which many copies or prints are prepared by conducting the imagewise exposure step only once.
The oldest technique of this method is disclosed in the specification of U.S. Pat. No. 2,812,709. According to this method, a toner image formed on a photosensitive layer by conducting the developing operation once is transferred in a divided manner onto copy sheets to obtain many copies (transfer repetition method). In this method, since the amount of the toner that can be applied by one developing operation is limited, the number of obtainable copies should naturally be limited, and if it is tried to obtain many copies beyond this limit, reduction of the image density and contrast cannot be avoided.
There has already been proposed a method in which development and transfer are repeated on one electrostatic latent image to obtain many copies or prints. For example, Japanese Patent Publication No. 30233/69 discloses a method in which a toner image is brought into intimate contact with a transfer sheet by an electrically conductive roller, a transfer voltage is applied between the toner image and the transfer sheet to transfer a part of the toner of the toner image to be transfer sheet and repeating the development and transfer while gradually increasing the transfer voltage to obtain many copies. Furthermore, Japanese Patent Publication No. 5056/75 discloses a method in which a latent image formed on a photosensitive layer is developed with a toner of the same polarity as that of the latent image, the formed toner image is brought into intimate contact with an insulating transfer sheet by an electrically conductive roller to transfer the toner image to the transfer sheet and this developing and transferring operation is repeated to obtain many copies. In these known methods, however, since a once formed electrostatic latent image should be subjected to the development repeatedly, there is involved the requirement that cannot industrially be satisfied, that is, the requirement that the development and transfer should be repeated without disturbance of the electrostatic latent images. Furthermore, in the former method, a troublesome operation of gradually increasing the transfer voltage should be carried out, and the latter method is defective in that a poorly printed area is formed in a broad black region and the image quality is insufficient, because the repelling development is carried out.
There also is known a method in which many copies are obtained by repeating charging, development and transfer after imagewise exposure conducted once, while utilizing the photomemory effect of a photoconductive photosensitive layer (the phenomenon in which the exposed area retains the electric conductivity even after the exposure). For example, photographic methods of this type are disclosed in R. M. Schaffert, "Electrophotography" (published in 1975 by Focal Press), D. J. Williams, Tappi, 56, No. 6 (1973), Eiichi Inoue, Lecture published on Nov. 11, 1971 at the 28th Meeting of the Japanese Society of Electrophotography and Japanese Patent Application Laid-Open Specification No. 117635/76.
In these methods utilizing the photomemory effect of a photoconductive photosensitive layer, no particular disadvantage is brought about when this photosensitive layer is used for electrostatic printing alone. However, in order to erase the photomemory effect of the photosensitive layer, it is necessary to conduct a troublesome operation of allowing the photosensitive layer to stand in the dark for a long time or heating the photosensitive layer by infrared rays or the like. When a photosensitive layer having such photomemory effect is applied to the ordinary electrostatic photographic reproduction process in which from many originals, corresponding copies are prepared, the copying speed is drastically reduced and this photosensitive layer is not suitable for commercial reproduction or printing.
U.S. Pat. No. 3,918,971 to Zweig discloses a photographic copying process in which a photoconductive insulating layer coated on a substrate is rendered uniformly non-conductive to make multiple copies by negatively charging the surface, the layer is next sensitized by positively charging the surface after which it is imaged and conventionally developed with electrostatically attractable material, the electrostatically attractable material is then conventionally transferred in offset fashion onto a copy sheet, surface charge is lost and the latent image is thereby degraded during the process of toning the imaged photoconductive surface and transferring the toner to the copy sheet, and repetitive copies can then be made from the latent image by passing a uniform positive charge over the layer to rejuvenate the latent image and repeating the developing and transferring steps each time.
In the photographic copying process of this type, in order to increase the copying speed, it is desirable to shorten the time for negative charging as much as possible. Moreover, in order to increase the image density, it is necessary to elevate the potential at the time of positive charging. Furthermore, in order to reduce the fog density, it is important that the residual potential in the light-exposed area should be reduced and this residual potential should always be controlled to a low level even when the steps of positive charging, development and transfer are repeated.